Skills

Mods add active skills (ModSkill) and passive skills (ModPassive). Both go on a tab of the game's skills menu, are learnt with ability points, and can require levels, attributes or other skills.

An active skill

An active skill starts as one of the game's - its targeting (a unit, a tile, an area, none), cast, cooldown, energy cost and sounds - and replaces its effect with yours. Every game skill has a generated class in StoneForge.GameSkills to inherit:

using StoneForge;
using StoneForge.GameSkills;

public class ShockBolt : ChainLightning
{
    private readonly Shocked _shocked;

    public ShockBolt(Shocked shocked) : base("shock_bolt")
    {
        _shocked = shocked;
        DisplayName = "Shock Bolt";
        Description = "10 shock damage, and its target Shocked for 4 turns.";
        Icon = "shock_bolt.png";
        Tab = "Stormcalling";
        Group = SkillGroup.Sorcery;
        Cooldown = 3;
        EnergyCost = 20;
    }

    protected override void OnCast(SkillCast cast)
    {
        Fx.Play(cast.Target, Sprite.s_weapondamage_electricity, new FxOptions { OffsetY = -14 });
        Combat.Damage(cast.Target, DamageType.Shock, 10, cast.Caster);
        Context.Buffs.Apply(_shocked, cast.Target, 4, cast.Caster);
    }
}

context.Skills.Add(new ShockBolt(shocked));

SkillCast has Caster, Target (the unit, or the game's mark on the tile aimed at; the caster for a skill without a target), X, Y, IsCrit (a miracle: a spell's critical cast) and Skill (its instance).

Any skill used

Skills.OnUsed runs as any skill has been used, the player's or a unit's, the game's or a mod's: its energy spent, its cooldown started, a spell's miracle or fumble rolled. Its skill, caster and target are kept by id.

Skills.OnUsed(context, cast =>
{
    if (Units.IsPlayer(cast.Caster.Instance) && cast.IsCrit)
        context.Log("A miracle!");
});

It isn't told of the plain actions that are skills in name only: moving, throwing an item, crafting, setting a trap, dousing, disarming, setting up a bed or a dummy, a shot's attack mode.

A passive skill

A passive is always on once learnt. It changes the character's stats, shown in the character sheet as the game's passives are, and reacts to fights:

using System;
using StoneForge;

public class StaticCharge : ModPassive
{
    private readonly Shocked _shocked;

    public StaticCharge(Shocked shocked) : base("static_charge")
    {
        _shocked = shocked;
        DisplayName = "Static Charge";
        Description = "+5% Crit Chance, and weapon hits may shock.";
        Icon = "static_charge.png";
        Tab = "Stormcalling";
        Set(BuffStat.CRT, 5);
    }

    protected override void OnHit(Attack attack)
    {
        if (Random.Shared.NextDouble() < 0.25)
            Context.Buffs.Apply(_shocked, attack.Target, 3, attack.Attacker);
    }
}

Where it goes, and what it takes

Shared by both kinds (ModSkillBase):

The skill's tooltip tells the player what's missing.